Synthetic membranes for hemodialysis, hemodiafiltration, or hemofiltration of blood usually are comprised of a hydrophobic polymer which is blended with a hydrophilic polymer to improve compatibility of the membrane with blood. Often polysulfone, polyethersulfone, or polyarylethersulfone are used as hydrophobic polymer, and polyvinylpyrrolidone is used as hydrophilic polymer.
WO 2004/056459 A1 discloses a permselective asymmetric membrane suitable for hemodialysis, comprising at least one hydrophobic polymer, e.g. polyethersulfone, and at least one hydrophilic polymer, e.g. polyvinylpyrrolidone. The outer surface of the hollow fiber membrane has pores in the range of 0.5 to 3 μm and the number of pores in the outer surface is in the range of 10,000 to 150,000 pores per mm2. The polyvinylpyrrolidone used to prepare the membrane in the working examples (PVP K30) has a weight average molecular weight of 50 kDa.
WO 2004/056460 A1 discloses a permselective asymmetric membrane suitable for hemodialysis, comprising at least one hydrophobic polymer, e.g. polyethersulfone, and at least one hydrophilic polymer, e.g. polyvinylpyrrolidone. The membrane allows passage of molecules having a molecular weight of up to 45 kDa in the presence of whole blood. The polyvinylpyrrolidone used to prepare the membrane in the working examples (PVP K30) has a weight average molecular weight of 50 kDa.
EP 2 113 298 A1 discloses a permselective asymmetric membrane suitable for hemodialysis, comprising 80-99 wt % of polyethersulfone and 1-20 wt % of polyvinylpyrrolidone, wherein the polyvinylpyrrolidone consists of a low molecular weight component having a molecular weight of below 100 kDa and a high molecular weight component having a molecular weight of 100 kDa or more. The spinning solution for preparing the membrane comprises between 12 and 15 wt % of polyethersulfone and 5 to 10 wt % of PVP consisting of a low and a high molecular PVP component.
EP 1 439 212 A1 discloses a hollow fiber membrane comprising polysulfone, polyvinylpyrrolidone and a cationic polymer. The polyvinylpyrrolidone preferably has a weight average molecular weight in the range of from 2,000 to 2,000,000 Da, more preferably 10,000 to 1,500,000 Da. Commercial products with a weight average molecular weight of 1,100,000, 45,000, 29,000, 9,000, or 29,000 have been used preferably. The polyvinylpyrrolidone content in the membrane preferably is in the range of 1 to 50 wt %, more preferably 1 to 10 wt %.
EP 1 913 964 A2 discloses a hollow fiber membrane type blood purification device comprising a bundle of hollow fiber membranes comprising a polysulfone resin and polyvinylpyrrolidone, wherein the hollow fiber membranes have an adhesion rate of a radical trapping agent of 80-300%, based on the dry weight of the hollow fiber membranes, and have a water content of not less than 40% and less than 100%, and have been sterilized with radiation. The molecular weight of the polyvinylpyrrolidone in the hollow fiber membrane is 10,000 to 2,000,000 Da, preferably 50,000 to 1,500,000 Da. The polyvinylpyrrolidone content in the membrane is 3 to 20%, preferably 3 to 10%, based on the total amount of the polymer.
EP 2 216 060 A1 discloses a porous hollow fiber membrane for treating blood having a membrane thickness of 25 μm to 40 μm, and providing fibrils having an average diameter of from 100 μm to 200 μm, wherein the ratio (To/Ti) of the average outside diameter (To) to the average inside diameter (Ti) is adjusted to not more than 2. The membrane comprises a polysulfone-based polymer and polyvinylpyrrolidone and has a gradient porous structure with an increase in pore size from the inside to the outside. The polyvinylpyrrolidone used preferably has a weight-average molecular weight of about 1,000,000 Da (PVP K80 to K90 grades). The PVP concentration in the membrane forming stock solution is adjusted so that the mixing ratio of PVP to polysulfone is 27 wt % or less, preferably 18 to 27 wt %, more preferably 20 to 27 wt %.
When polyvinylpyrrolidone having a relatively low molecular weight (like PVP K30) is used in the preparation of a membrane, a large amount of the polyvinylpyrrolidone initially present in the membrane is leached from the membrane during washing. On the other hand, membranes prepared using only polyvinylpyrrolidone having high molecular weight (like PVP K80 or K90) tend to show lower clearance of solutes like urea or vitamin B12 from blood and higher protein loss.